1. Field of the Invention
This invention relates to analog-to-digital converters, and more particularly, to sigma-delta type analog-to-digital converters implemented with superconducting circuit elements.
2. Description of Related Art
Sigma-delta converters are a class of analog-to-digital converters. The basic sigma-delta converter utilizes an integrator to which the analog signal to be converted is applied. A single rough comparator operating at high speed converts the output of the integrator to a single bit digital signal. The comparator is combined with a digital-to-analog converter to form a quantizer. A digital filter converts the high speed single bit output of the quantizer into a multi-bit digital output. The converter also includes a feedback loop including the comparator, the digital-to-analog converter and the integrator. The quantizer samples the integrator output at a rate many times the Nyquist rate. The rate of the multi-bit output signal is a function of the sampling rate and the number of bits in the output signal. The feedback in the sigma-delta converter integrates the error in the least significant bit thereby shifting quantization noise, which limits the dynamic range in conventional analog-to-digital converters, to frequencies above the frequency of the analog input signal. The chief advantage of sigma-delta converters is that they substitute high speed digital signal processing for the high precision analog circuits required in conventional analog-to-digital converters.
Our commonly owned patent application Ser. No. 07/710,856 filed on Jun. 6, 1991, now U.S. Pat. No. 5,140,324, discloses a sigma-delta converter implemented with superconducting elements and operated with GHz sampling rates to provide high resolution for megahertz signals. This superconducting sigma-delta converter utilizes a superconducting inductor as the integrator, a Josephson junction as the quantizer and a superconducting Quantum Interference Device (SQUID) to generate GHz rate sampling pulses. When the sum of a sampling pulse plus the inductor current generated by the superconducting inductor exceeds a critical current, the Josephson junction generates a voltage pulse which represents a digital "ONE" output. The voltage pulse also provides feedback to the superconducting inductor. This feedback is very precise and stable as each voltage pulse generated by the Josephson junction is a flux quantum.
Our commonly owned patent application Ser. No. 07/807,040 filed on Dec. 12, 1991, now U.S. Pat. No. 5,198,815, discloses a two loop superconducting sigma-delta analog-to-digital converter which includes a first superconducting inductor to which the analog signal is applied. A resistor converts the current in the first inductor to a voltage which is applied to a second superconducting inductor. The current in the second inductor, which increases quadratically with time, is applied to an overdamped Josephson junction which kicks back a single quantum voltage pulse each time its critical current is exceeded. This voltage pulse reduces the current in the second inductor and serves as a digital "ONE" output. The pulses are also applied to an underdamped Josephson junction in a feedback pulse generator which latches at its gap voltage for the remainder of a half cycle of an ac bias current. This provides a voltage source for the primary of a superconducting transformer having a mutual inductance which provides sufficient flux in the secondary to cause a SQUID to generate in response to each pulse from the quantizer a selected number of feedback pulses which are applied to the first inductor.
Sigma-delta analog-to-digital converters substitute high-speed digital signal processing for high-precision analog components to achieve high-dynamic range. Commercially available silicon sigma-delta modulators give 18-24 bits in the audio band. Sigma-delta analog-to-digital converters use oversampling and digital filtering to obtain this performance. Semiconductor sigma-delta analog-to-digital converters are limited to sampling at MHz rates by the requirement of providing twenty-bit accurate feedback. Consequently, 18-24 bit accuracies have only been obtained for signal bands of a few kHz.
Semiconductor sigma-delta analog-to-digital converters were developed for the high-performance audio market. Audio signals are inherently base band, having a band from near d.c. to about 25 kHz. Their function is naturally low-pass. In the typical sigma-delta analog-to-digital converter, shown in FIG. 1, the anti-aliasing filter 2 that precedes the modulator 4 is low-pass, and so is the final digital filter 6. Performed synchronously by means Of sampling clock 8, an input analog signal is converted to a digital signal at the Nyquist rate.
Superconductor sigma-delta modulators employ quantum mechanical feedback to enable sample rates in the 1-100 GHz range. This makes it possible to obtain 18-24 bits of dynamic range for signal band widths up to 25 MHz.
Superconductor sigma-delta analog-to-digital converters open up the possibility of digitizing wide portions of the rf spectrum, for example, 16 bits on a 60 MHz signal band, for high dynamic range digital radio receivers. These signals are not inherently base band. Furthermore, it is not a simple matter to mix these signals to base band. Practical 90 dB phase-linear image rejection filters with 400-1000 MHz center frequencies have large shape factors. When the filtered signal is subsequently mixed to base band, a large frequency band extending from d.c. contains aliased information. This reduces the effective bandwidth of a base band analog-to-digital converter to a fraction of its potential.
Bandpass sigma-delta modulators are a relatively new idea intended to apply sigma-delta techniques to rf receivers. Sigma-delta modulators use oversampling and active filtering to develop a high signal-to-noise ratio in the signal band. Typically, this is a low pass band. In bandpass modulators, the maximum suppression of quantization noise must occur at an intermediate frequency, such as 150 MHz. Consequently, there is a need for a sigma-delta modulator for a band pass operation.